1. Field of the Invention
The present invention relates to a double exhaust pipe for an engine, and in particular, to an improvement of a double exhaust pipe for an engine, comprising an inner pipe shell, an outer pipe shell in which the inner pipe shell is accommodated, the inner pipe shell being secured at one end thereof to the outer pipe shell and slidably fitted at the other end to an inner peripheral surface of the outer pipe shell, and a heat-insulating space provided between the inner and outer pipe shells to extend from the one end to the other end of the inner pipe shell.
2. Description of the Related Art
There is such a double exhaust pipe for an engine, as disclosed, for example, in Japanese Utility Model Publication No. 2-40249 wherein a bulged portion is formed at the other end of an inner pipe shell, and has a cylindrical surface slidably fitted to an inner peripheral surface of an outer pipe shell. In such double exhaust pipe, a drop in temperature of an exhaust gas flowing through the inner pipe shell can be prevented by the presence of the heat-insulating space between the inner and outer pipe shells, thereby enhancing the exhaust emission control function of an exhaust emission control device connected to a downstream portion of the double exhaust pipe. In this case, a thermal elongation of the inner pipe shell is absorbed by slipping the bulged portion at the other end, i.e., the free end of the inner pipe shell on the inner peripheral surface of the outer pipe shell.
However, the double exhaust pipe for the engine generally has a bent portion. For this reason, when the inner pipe shell is thermally elongated, one end and the other end of the inner pipe shell are thermally elongated in different directions. Due to this, the bent portion is further bent and hence, an inclination occurs on the side of the free end of the inner pipe shell. When the free end side is inclined, an end edge of the bulged portion of the free end bites the inner peripheral surface of the outer pipe shell to increase the sliding resistance to the outer pipe shell, whereby an excessively large thermal strain is generated in the inner and outer pipe shells, or a failure of contact occurs between a portion of the bulged portion and the inner peripheral surface of the outer pipe shell to generate a chattering sound or vibration sound. The foregoing has been found by the present inventors.
Accordingly, the present invention has been proposed based on the mentioned finding and it is an object of the present invention to provide a double exhaust pipe for en engine, wherein even if the free end is inclined upon the thermal elongation of the inner pipe shell, no failure of contact occurs between the bulged portion at the free end and the inner peripheral surface of the outer pipe shell and hence, the bulged portion can always be smoothly slipped on the inner peripheral surface of the outer pipe shell.
To achieve the above object, according to a first aspect and feature of the present invention, there is provided a double exhaust pipe for an engine, comprising an inner pipe shell, an outer pipe shell in which the inner pipe shell is accommodated, the inner pipe shell being secured at one end thereof to the outer pipe shell and slidably fitted at the other end thereof to an inner peripheral surface of the outer pipe shell, and a heat-insulating space provided between the inner and outer pipe shells to extend from the one end to the other end of the inner pipe shell, wherein the inner pipe shell has a bulged portion formed at the other end thereof, the bulged portion having a spherical outer surface slidably and oscillatably fitted to an inner peripheral surface of the outer pipe shell.
With the first feature, when the other end, i.e., the free end of the inner pipe shell is inclined upon thermal elongation of the inner pipe shell, the bulged portion at the other end thereof is oscillated with little resistance in response to such inclination, and the state of the bulged portion fitted to the outer pipe shell is maintained constant. Therefore, the slipping of the bulged portion on the inner peripheral surface of the outer pipe shell is not impeded, and the generation of a thermal strain in the inner and outer pipe shells can be inhibited effectively. No failure of the pressure contact of the bulged portion with the inner peripheral surface of the outer pipe shell can be brought about and hence, the generation of a chattering sound or vibration sound can be also prevented.
According to a second aspect and feature of the present invention, in addition to the first feature, the bulged portion is comprised of three or more crests arranged in a circumferential direction of the inner pipe shell with valleys which are interposed between adjacent ones of the crests and spaced from the inner peripheral surface of the outer pipe shell.
With the second feature, the contact of the entire periphery of the bulged portion of the inner pipe shell with the outer pipe shell can be avoided, while ensuring the concentricity of the bulged portion and the outer pipe shell, thereby inhibiting the heat transfer from the inner pipe shell to the outer pipe shell to the utmost.
According to a third aspect and feature, in addition to the first or second feature, wherein each of the crests has a radius of curvature of an outer surface thereof in the circumferential direction of the inner pipe shell determined smaller than an inside diameter of the outer pipe shell.
With the third feature, the bulged portion of the inner pipe shell can be brought into generally point contact with the outer pipe shell, thereby effectively inhibiting the heat transfer from the inner pipe shell to the outer pipe shell.
According to a fourth aspect and feature of the present invention, there is provided an exhaust manifold for an engine, comprising a plurality of exhaust pipe branches each formed of the double exhaust pipe according to any of the first to third features.
With the fourth feature, it is possible to provide an exhaust manifold which has a high heat-retaining property and in which a thermal strain is little produced.